Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
In the face of global problems associated with the lack of sustainable and renewable energy sources and environmental pollutions caused by fossil fuels, fuel cells have long been expected to contribute to the solutions to these daunting problems by helping the production of electricity from fuels such as hydrogen. Unfortunately, however, the electrodes (both cathode and anode) in many conventional fuel cells are composed of the expensive and less earth abundant noble metal platinum, which serves as the electrocatalyst. This remains one of the bottlenecks currently plaguing fuel cells from finding a wide range of applications. The second major problem in fuel cells remains the inherent poor efficiency of the oxygen reduction reaction (ORR)— one of the redox reactions that has to take place at one of the half cells within fuel cells. Even with the use of platinum-based electrodes or electrocatalysts, this reaction is very sluggish or has high overpotential. For instance, while hydrogen oxidation reaction (HOR) on the anode side of the fuel cells typically occurs with overpotential as low as 50 mV, the ORR often has an overpotential as high as 500-600 mV, even with platinum-based electrocatalysts (Norskov et al. (2004) J. Phys. Chem. B, 108:17886). The ORR is, therefore, mainly responsible for the limited current density and reduced cell voltages obtained from fuel cells. Thus, a massive improvement in fuel cells requires not only finding electrocatalysts based on sustainable and earth-abundant elements but also rationally designing and synthesizing noble metal-free and inexpensive electrocatalysts capable of performing ORR as efficiently as, if not better than, platinum.
Recent efforts to obtain replacements of platinum and its congener metals for ORR have resulted in some new options, including some N-doped carbon-based materials, which showed promising catalytic activity for ORR (Gupta et al. (1889) J. Appl. Electrochem., 19:19; Gojkovic et al. (1999) J. Electroanal. Chem., 462:63; Matter et al. (2006) J. Catal., 239:83). These and other studies have also suggested that further improvements on the catalytic activities of carbon-based materials are possible by doping the latter with heteroatoms such as boron, phosphorus or sulfur. However, the mechanisms by which these heteroatom-doped carbon-based electrocatalysts improve ORR as well as the functions of the heteroatoms in these systems are not yet well-understood (Wang et al. (2012) Angew. Chem. Int. Ed., 51:4209). Nevertheless, many recent studies on nonmetallic heteroatom-doped carbon materials indicate that superior activity toward ORR may require multifunctional catalytic systems (Wang et al. (2011) Angew. Chem. Int. Ed., 50:11756).
Besides nonmetals, metallic dopants, mainly cobalt ions, have also been demonstrated to improve the electrocatalytic activity of N-doped carbon-based materials. Although some authors suggested that metal ions that remain coordinated to the nitrogen atoms of the N-doped carbons act as the active sites, there are other recent studies suggesting otherwise (Lefevre et al. (2000) J. Phys. Chem. B, 104:11238; Sawai et al. (2004) J. Electrochem. Soc., 151:A682; Lefevre et al. (2009) Science, 324:71; Subramanian et al. (2009) J. Power Sources, 188:38; Kothandaraman et al. (2009) Appl. Catal. B: Environ., 92:209; Kundu et al. (2009) J. Phys. Chem. C, 113:14302). For instance, it has been suggested that the added metals affect only the carbonization process of the molecular precursors into better N-doped carbon electrocatalysts by promoting the formation active nitrogen catalytic centers within the material (Oh et al. (2012) J. Power Sources, 212:220). Cobalt was also shown to enhance ORR when it was added in the form of nanostructures, such as Co, CoO and Co3O4 nanoparticles, onto graphene oxide, carbon nanotubes or conducting polymers. This provides further evidence for why having cobalt or metal dopants into carbon-based materials is considered to improve the electrocatalytic activity of the latter toward ORR (Bashyam et al. (2006) Nature, 443:63). Nevertheless, regardless of how metal dopants enable improvements in electrocatalytic activity of carbon-based materials, metal-free catalysts are still preferable for ORR, especially if the latter show good enough electrocatalytic activity. This is mainly because ORR is often carried out under acidic or basic media, and metals such as cobalt can easily leach out from the electrodes' (electrocatalysts') surfaces over time under these conditions, if not quickly, and result in reduced electrocatalytic activity and shorter shelf-lives to the electrocatalyst (Wang, B. (2005) J. Power Sources, 152:1).